Transition of laminar pre-mixed flames to turbulence - induced by sub-breakdown applied voltage Biswa N. Ganguly Aerospace Systems Directorate, Air Force Research Laboratory WPAFB OH USA and Jacob Schmidt Spectral Energies, Dayton OH
Ionic Wind Effects - Background Potential role of sub-breakdown electric fields to augment/modify combustion kinetics and flame fluidics. Lead to overall increase in stability, combustion efficiencies Many experimental results reported over past sixty years. Direct experimental evidence or robust modeling of exact mechanism by which sub-breakdown applied voltage modifies flame fluidics and kinetics are currently not unequivocally known. Lift-off and blow-off limits of flames extended by AC\DC field induced ion wind by Kim et al. Adiabatic burning velocity increase by DC imposed electric current through thermal power release and/or direct chemical reaction rate proposed by van den Boom et al. Kim M, Ryu S Won S, Chung S() Combustion and Flame 7():7- van den Boom J, Konnov A, Verhasselt A, Kornilov A, de Goey L, Nijmeijer H (9) Proceedings of the Combustion Institute :7-
Ionic Wind Body-Force The ion and electron volumetric forces on a neutral molecule can be represented by the two equations below: f f f i i e n m i n e e i e im m Using the drift-diffusion equations and charged particle mobility expressions, it is possible to rewrite these equations as the following: Assuming: u em i u e( n n ) E kt n i e i kt n Average number density of positive ions for a near stoichiometric C H 8 /air flame at atmospheric pressure 6 m - Maximum possible gradient exits over. reaction zone thickness. Gradient terms provide a body force of about.n/m e e Boeuf, J. and Pitchford, L., J. Appl. Phys., Vol. 97,, pp. 7:-.
Wire Loop Cathode with Burner Floating Flame with wire loop as cathode. In figure (a) the loop was placed above the floating burner, whereas in figure (b) the loop was placed above the floating burner. As the voltage is applied up to a positive.6 kv value, there is no observable change in the flame height or stability. Images are taken with a 8 nm band-pass filter in place to allow the imaging of the OH * chemiluminescence, which can be used as a marker for the flame front.
Burner Head Cathode Flame with the burner head grounded. In this case the wire loop has been completely removed, and the sheath is formed at the burner head (preheat zone). ms exposures Difference in flame heights with and without an applied voltage suggests an effective flame speed increase of approximately %. As the voltage is applied, there is a significant change in the flame height and stability. Images were taken with a 8 nm band-pass filter. The blurring seen when the voltage is applied is due to the rapid movement of the flame front, and the long time exposure of the camera.
Sheath Formation Sheath Edge Positive Ions Electrons The applied positive bias sets up a cathode sheath at the flamemetal surface boundary. The electrons are repelled from the cathode and the positive flame ions are attracted to it. This creates an area in the flame with a net positive charge. As the positive ions reach the cathode (burner head or wire loop) Cathode some of them will collide with the neutrals imparting momentum transfer. The positive ions are accelerated as they approach the sheath edge, causing them to reach the Bohm velocity.
Ionic Wind Effects at the Sheath The effects of both the density gradients and the cathode sheath would be present near the ring electrodes, where we find no measurable effects of ionic wind. Near the burner head, the ionic wind-induced body-force on the flame should be dominated from the cathode sheath. The average volumetric force is given by, f Een i Newton m - for E=. v/m, and n 6 m - i Multiplying the volumetric force by the sheath thickness (taken to be electron Debye length, D ) gives an expression for the average pressure change across the flame due to the ionic wind effects. p Een i D - Pascal Can such a small pressure change impact flame structure?
Flame Structure Modification By Applied Voltage Broadband images of a propane/air flame at atmospheric pressure with an equivalence ratio of.. Starting from left to right the voltage is increased (as indicated above the flame) to show flame structure changes with an applied electric field vector opposing flow velocity.
Temporal Scan of Hz PIV and OH Chemiluminescence ms ms 6ms 8ms +kv ms pulsed @ Hz applied downwards over gap Maximum velocity decrease (~%) occurs at end of voltage pulse Track perturbation origination to very near-burner head region at voltage onset (t = ms) Time-stitched PIV data assumes pulse to pulse reproducibility
KHz Broadband Chemiluminescence Flame structure modifications are reproducible up to ms and shows stochastic behavior beyond ms.
Heat Release Modulation via OH* and Broadband Emissions vs Current The frequency spectrum of the OH* emissions from the flame are essentially identical to the that obtained from the bias circuit s current waveform. The OH* emissions have been shown to correctly directly with local heat release. These results suggest that conductivity probes might offer an alternative to optical diagnostic approaches for detection of the onset of pulsating, oscillatory or otherwise unstable combustion conditions.
Flame Turbulence Induced by Ionic Wind Effects Both high speed imaging and FFT data show the pre-mixed laminar flame is transitioning to turbulence. Time stitched PIV data captures only time averaged flame structure modifications. A high repetition rate PIV measurement can capture the true flame structure modification induced by chemi-ion drift current induced body force exerted to the flame. 6 KHz rep rate PIV measurement is performed to obtain real time fluid flow modification using dual cavity frequency doubled Nd:YAG laser.
Voltage [kv] Current [µa] Experimental Schematic... - Time [ms] Oscillosope e Seeder kω A MFC MFC + IGBT Pulse Genera tor DG6 HV Pow er Sup ply Propane tank Air compressor ms voltage pulse duration @ Hz kv, µa,. Watt
High Rep Rate PIV Experimental Setup Laser imaging Burner layout
Typical PIV Data Photron SA (Bit CMOS) camera imaging @ 6kHz with 768 x 768 resolution 8 μs exposure times with. /pixel spatial resolution x region of interest imaged Collected and processed initially with DaVis 8. software from LaVision Inc. Sequential cross-correlation scheme with double passes of 6x6 and x pixel regions rejecting areas of too high seed density or low contrast (8 x 8 vector field) Displaying sample data with delay shown relative to onset of ms applied voltage pulse ms ms ms m/s m/s m/s
6 khz PIV Data The six different PIV velocity vector sets show different temporal positions of the same flame subjected to ~.kv positive polarity applied voltage pulse of milliseconds (ms) in duration, from an anode (just out of view) placed above the flame and above the grounded burner body.. ms. ms 9.66 ms 8. ms 8.66 ms 8.8 ms Four regions are highlighted which show the advantage of high rep rate PIV data showing real time evolution of flame fluidics. Highlighted regions in 8. and 8.66 ms data show velocity change by +.7 m/s; 8.66 and 8.8 ms data show velocity change by -.6 m/s.
Voltage [kv] Velocity Histograms at Three Locations - ms - ms - ms - ms
Plots of V y / y for Two Locations
Conclusion From khz PIV Data Investigated fluidic response of flame perturbations caused by ms-wide voltage pulses below self-sustained breakdown. Observed. W electrical input power have very significant effect on. kw flame structure. High electrical efficiency is due to no ionization energy cost with sub-breakdown electric field. Relatively small amount of electrical power can cause an otherwise stable, laminar flame to highly unstable behavior/ transition to turbulence. Higher V y / y in center of flame suggesting highly strained burning condition. Velocity histogram shows the onset of induced turbulence beyond ms, consistent with high speed imaging and FFT. Observed benefits include lateral flame spreading and reduced flame height resulting in modified heating zones near burner surface with potential for positive implications such as improved flame holding. Response after voltage is shut off suggests that recovery mechanism is mostly fluidic in nature. High speed measurements are essential for quantifying turbulent phenomena.
Acknowledgements Wish to than Stan Kostka for his help with the data taking. Thanks to Sukesh Roy and Jim Gord for use of their new PIV systems. Work was supported by Chiping Li, AFOSR.
6 khz Repetition Rate PIV Data Velocity Vectors 6 Velocity Vectors 6 Velocity Vectors 6 ms 8 ms ms - - - - - - - - - - - - 6 ms Velocity Vectors 6 ms Velocity Vectors 6 ms Velocity Vectors 6 - - - - - - - - Velocity Vectors 6 - - Velocity Vectors 6 - - Velocity Vectors 6 ms 6 ms ms - - - - - - - - - - - -
OH PLIF Data Five consecutive single shot OH PLIF data are taken at the same time delay. Shot to shot variation of OH PLIF after 8 ms delay is indicative of stochastic flame fluctuations. Agrees with the PIV data. (a) OH PLIF Set-up (b) Combined OH PLIF and flame chemilumenisence